There are several structural groupings that are important in inorganic non-metallic technology. Many are widely used in such diverse applications as refractories, as ferroelectric devices, as inorganic pigments and the like. These compounds are typically mixed metal oxides although they may also contain sulfur, carbon or the halogen elements. Of particular interest is the use of mixed metal oxides as inorganic pigments for ceramics, paints and plastics. Typically, in the pigment industry, classes of pigments that are well recognized are described in a publication of the Dry Color Manufacturers Association, discussed in greater detail hereinbelow. For these uses, it is desirable to produce pigment particles with very small uniform particle sizes, which are phase pure and defect free.
Typically, mixed metal oxide inorganic pigments are commercially, although not exclusively, produced by either a) a solid state reaction process involving the wet or dry blending of various metals, oxides or salts, subsequent calcination at elevated temperatures, to ensure that the desired reaction occurs, followed by comminution (or deagglomeration) to the desired size and washing and drying (if required, to remove unwanted salts) or b) chemical precipitation which may be followed by calcination and subsequent grinding (or deagglomeration) to the desired size and washing and drying (if required to remove unwanted salts), or c) combinations of both processes.
Modern practice attempts to maximize dry process options in the interests of economy and energy efficiency by batching and dry blending raw materials prior to calcination. The raw materials used are fine powders typically with particle sizes in the range of 0.2 to 50.mu.. It is normally not the purpose of the dry blending process to reduce the particle sizes of the constituent powders, but seeks to distribute them evenly. However, dry blending cannot generally produce raw batches that are homogeneous on a submicron scale. The calcinations are typically 0.1 to 24 hours in length to allow for large scale production; however, this is often insufficient to permit complete diffusion of the active species and reaction of the coarser or more refractory raw materials. Calcination can be achieved in periodic, intermittent kilns, or continuous rotary or tunnel kilns. Final size adjustment is achieved by either wet or dry comminution devices which might include, ball milling, attrition milling, micropulverization or jet milling. Wet comminution is followed by a drying operation or, a filter, wash and drying operation.
The typical pigment manufacturing process described above causes a number of significant problems for the production of high quality pigments. Some common difficulties are: achieving complete reaction; production of a single phase product; production of fine sized particles; production of narrow particle size distributions; formation of aggregates and large particles which are difficult or impossible to mill down to the desired size; and, elimination of grit and large particles (&gt;2.mu. or &gt;10.mu., depending on the pigment application).
It is also common practice in the pigment industry particularly in the case of zircon pigments to assist the high temperature reactions by the use of salts, (sometimes called fluxes or mineralizers) which melt, form eutectics or a reactive vapor phase which is conducive to the mutual migration or diffusion of the active species. Their use is largely based on experience because generally there is no reliable manner of predicting which particular mineralizer or combination will enhance the formation of a given color, or amount thereof. Mineralizers are typically employed to enhance liquid phase formation, eutectic melt systems and vapor phase reactions. Such mineralizers are typically fluorides, chlorides, sulfates, oxides and other salts which might be used singly or in multiple combinations. Depending upon the application of the pigment it is frequently necessary to wash the finished pigment to remove residual salts or mineralizers.
The art and literature demonstrate the desirability of obtaining and employing small, uniform particle sizes for pigment applications as well as techniques involving precursors and seeding in order to provide improved particles and/or properties.
The importance of a pigment's particle size with respect to its optical performance is discussed, for example, by W. R. Blevin and W. J. Brown in an article entitled "Light-Scattering Properties of Pigment Suspensions", Journal of the Optical Society of America, Vol. 51, No. 9, September 1961. The overall importance of the particle size of a material with respect to its interaction with electromagnetic radiation can be found in the book written by C. F. Bohren and D. R. Huffman entitled Absorption and Scattering of Light by Small Particles, John Wiley & Sons, 1983. The importance of the pigment's particle size and shape to the rheological performance of the pigment in liquid systems is discussed, for example, by P. Kresse in an article entitled "Influence of the particle size and particle form of inorganic pigments on change of shade in coloured paints and lacquers", Journal of the Oil Color Chemists Association, Vol. 49, 1966.
U.S. Pat. No. 4,752,341, for instance, describes a pigment system for paper which employs zeolite and TiO.sub.2. To aid the paper making process, the patent teaches the use of zeolite having an average particle size of less than 3.mu. and a crystallite size of less than 1.mu.. If the particle and crystal size are much larger, the quality of the paper is reduced. While recognizing this necessity, the patent does not provide a means for manufacturing small particle and crystal sizes.
U.S. Pat. No. 4,767,464 is directed toward carbonate-containing mineral materials, such as chalk, limestone, marble, synthetic CaCO.sub.3 and dolomite. Such materials have several uses, including pigments, and preferably have a small mean particle diameter of 0.5 to 2.5.mu., obtained by dry grinding.
U.S. Pat. No. 4,882,301, owned by the Assignee of record, is directed toward glass enamel systems designed to be fused onto a glass substrate at temperatures of between 1000.degree. F. (538.degree. C.) and 1350.degree. F. (732.degree. C.). The glass fraction of the system is a lead borosilicate glass. A feature of the glass enamel system is the presence of a crystallizing amount of a precursor of cadmium or zinc orthosilicate and/or cadmium or zinc metasilicate. The crystallizing amount of the precursor is that amount sufficient to produce crystallized cadmium silicate upon firing to harden the melt of glass enamel. These systems ultimately contain inorganic pigments or opacifiers to impart a desired black or dark gray band on glass employed on automobiles.
The use of alpha alumina seed crystals to lower the transition temperature of a sol-gel derived boehmite powder and to control the sintering of a ceramic body made from this mixture is described by Messing et al in "Seeded Transformations for Microstructural Control in Ceramics", Chapter 28, pp 259-271, Science of Ceramic Chemical Processing, Wiley-Interscience, 1986, Hench and Ulrich Editors. This method for the preparation of sintered ceramic bodies is covered by European patent 172764. The use of alpha-Iron (Hematite), which is isostructural to alpha-alumina as a seed crystal instead of alpha-alumina is discussed by Messing et al in "Controlled Chemical Nucleation of Alpha Alumina Transformation", Science of Ceramics, 14, pp 101-106, 1988 and in "Transformation and Microstructure Control in Boehmite-Derived Alumina by Ferric Oxide Seeding", Advanced Ceramic Materials, Volume 3, Number 4, pp 387-392, 1988.
The use of zircon particles to increase the rate of the reassociation of plasma dissociated zircon is described by McPherson et al in "The Reassociation of Plasma Dissociated Zircon", Journal of Material Science, 20, pp 2597-2602, 1985. It should be noted that the plasma dissociation process breaks down the zircon crystal into ultra-fine (&lt;0.1.mu.) zirconia particles and an amorphous Silica glass.
Finally, a preparation of zircon powder is described in an article by Kobayaski et al entitled "Preparation of ZrSiO.sub.4 Powder Using Sol-Gel Process (1)--Influence of Starting Materials and Seeding" Journal of the Ceramic Society of Japan, Int. Ed., Vol. 98 (June 1990). More particularly, the authors investigated the effect of temperature, heating rate and the addition of ZrSiO.sub.4 seed crystals on preparation by the sol-gel process to obtain high purity zircon powder. Generally, they found that seeding allowed the initiation temperature of zircon formation during calcination to be lowered by about 212.degree. F. (100.degree. C.) to 2192.degree. F. (1200.degree. C.). When calcination was then increased from 2912.degree. F. (1600.degree. C.) to 3002.degree. F. (1650.degree. C.), an almost pure, single phase zircon powder was obtained.
Typical commercial pigments are produced by mixing raw materials in the form of oxides, carbonates, hydroxides, hydrates, oxalates or the like, in wet or dry form, and then firing the mix at high temperatures in furnaces of varying construction and means of material transport. Two common methods are to load the mixed material into large crucibles, known in the industry as saggers, and fire them either stationary or on a moving slab or, to fire the material by feeding the material into a rotating tube furnace. It is difficult in either of these solid state processes to synthesize a material that is well crystallized, phase pure, with a controlled, fine particle size which has a narrow particle size distribution. The situation is even more difficult when one considers the economics of the situation. Typically the optimum particle size of the pigment is smaller than the particle size of commercially viable raw materials. It is extremely difficult to form high quality ultra-fine particles out of typical, commercially available, inexpensive larger raw materials. The material typically produced with the above processes and raw materials must often go through extensive grinding operations to reduce their size to the proper value without the use of ultrafine raw materials. Also, the very act of extensive grinding produces broad particle size distributions, which can be disadvantageous.
None of the foregoing technology describes a method for obtaining small and uniform particle sizes and shapes of mixed metal oxide powders from a solid state reaction. Such a method would have significant advantages over current technology which requires that relatively "large" crystals be ground to desired size after manufacture and introduces the possibility of contamination by the grinding media. Not only is grinding an additional step, adding to the cost, but the quality of the product may suffer. Growth of a small crystal would also allow the crystalline structure to remain intact and impart greater stability with respect to weatherability, resistance to attack with the suspending media, e.g., ceramics, glass, plastics, paints and the like.